A magnetostrictive sensor is typically used for detecting a torque or a load. In many magnetostrictive sensors, a magnetostrictive portion containing a magnetostrictive material is formed on an outer peripheral surface of a member (substrate) on which a torque or a load acts. In a magnetostrictive sensor having such a structure, a coil is disposed to surround the substrate on which the magnetostrictive portion is formed.
The above-described structure of the magnetostrictive sensor causes a magnetic permeability of the magnetostrictive material of the magnetostrictive portion to change when a force acts on the substrate on which the magnetostrictive portion is formed. Accordingly, the impedance of the coil disposed to surround the magnetostrictive portion changes. The magnetostrictive sensor detects the change in the impedance of the coil, thereby detecting a torque or a load.
Patent Document 1 discloses a torque sensor using a magnetostrictive material made of a Fe—Ni binary alloy. A structure disclosed in this document is intended to increase sensor sensitivity by determining the composition of Fe—Ni in such a manner that a preferable relative permeability and a preferable magnetostriction constant are obtained.
Patent Document 2 discloses a method for forming a magnetostrictive portion on a substrate. In this method, the substrate is immersed in a plating solution with a predetermined portion of the substrate being masked with a masking member. Thereafter, a current is caused to flow in the plating solution so that a plating film (magnetostrictive portion) is formed on an unmasked portion of the substrate. The magnetostrictive portion disclosed in Patent Document 2 includes a plurality of parallel lines (magnetostrictive lines).